Section IX is not a construction code, it simply provides general direction on how to qualify the welder and the welding procedure as well as the brazer and brazing procedure.
For specific "do's and don'ts" one mush refer to the construction code, i.e., Section I, B31.3, etc.
As already indicated, consult the code of construction for specific restrictions on Procedure Qualification. ASME Sec. IX by itself is a very "liberal" code to qualify a WPS with when compared to some others.
Joint Details in general are non-essential variables for procedure qualification for many processes. See Tables QW 252 thru 256 and observe that all joint design variables are non-essential.
Codes of construction that apply may impose limits on backing that must be observed during fabrication. this limits are usually not related to qualification of the WPS.
Google MIL-STD-22D.
Look at the B2V.3 Joint detail, figure 5.
We leave a small gap then lay down the root pass and few others.
When we turn the plates over, we grind out the back side of the first root pass then lay in a root pass and call that a backed weld.
There is a difference in a back weld and a backing weld.
A back weld is performed after back gouging as indicated by Dividstecker.
A backing weld serves the same function as a backing bar.
The back weld is the "last" weld deposited to "fill" the groove that results when the second side is back gouged to remove any unfused material or weld in the root that might be compromised.
The backing weld is the first weld bead deposited that supports the root bead while it is in the liquid state and cools and solidifies. A typical application could be submerged arc welding. When the joint is assembled, there may be some root opening (gap) between the two members. A backing weld is deposited to "seal" the joint so the granular flux doesn't simply flow through the root opening. So, the backing weld seals the joint to keep the flux in place. Once the root bead is deposited, the backing weld has served it purpose. If it was deposited on the root side of the joint, it can be removed by back gouging and back welded to ensure the weld is sound.
Once the procedure is qualified, the production welds can be made with or without the backing weld or with or without backing because the backing is a nonessential variable.
As per ASME IX,backing welding(or)back side welding is non essential variable(for WPS).
But in PQR,as we have to record actual condition of coupon welded,you need to specify whether back side welding done or not,with sketch of joint details.
